The economic use of various heating apparatus is an important goal of the building controls industry. However, until now, it is believed that a dual fuel control approach has not been provided to ensure minimizing energy costs to the consumer. Prior art systems controlling heating in interior zones with more than one heating unit have included, for example, systems as is disclosed in Thompson, et al., U.S. Pat. No. 4,716,957. Thompson, et al. discloses an air conditioning system comprising a heat pump and an auxiliary heater wherein a main controller controls an air duct multizone system. The controller includes a microprocessor and a memory. It receives information regarding the operating costs of the heat pump and of the auxiliary heater, which may be a gas burner, to ensure their cost effective use. Although the system checks simple gas energy costs against electric energy costs as a function of ambient temperature, it does not minimize electrical demand cost, and although demand charges are an important consideration in any heating environment, the system does not calculate a break-even demand set point.
Jones in U.S. Pat. No. 4,645,908 discloses a residential heating, cooling and energy management system that includes a heat pump and a microprocessor which is designed for use with a thermal storage device and does not switch between alternate fuel energy sources based on cost. Benton in U.S. Pat. No. 4,378,763 discloses a multistage thermostat using multi-rate integral action and exponential set point change for a heat pump and gas furnace combination. Benton's system reduces control point offset (known as droop). The signal processing means disclosed by Benton includes an integrator channel and a proportional constant channel in parallel. Benton's invention is aimed at ensuring most efficient restoration of a building to its daytime temperature after the building temperature has dropped to its set back temperature. Benton does not account for factors including demand cost, break even demand set point, gas cost, compared to electrical energy cost, predicted non-HVAC loads and heating loads. Thus, the system is not intended to reduce total utility bills, but instead to maximize use of the heat pump during the recovery period.
Beckey, et al. in U.S. Pat. No. 4,702,413 discloses a temperature control system using a single ramp rate control of a multi-plant environmental unit. A heat pump with an auxiliary electric heater is disclosed. A microprocessor and memory allow for energy savings through the set back functions. However, Beckey assumes that later stages are more costly than first stages, which is not always the case. Beckey provides no method to operate the auxiliary heat source without the heat pump.
Harshbarger, Jr., et al. in U.S. Pat. No. 4,627,484 discloses a heat pump control system utilizing auxiliary heat from a fossil fuel furnace. A microprocessor is utilized which includes economy of operation and the controls. Harshbarger, Jr. does not disclose any means for electrical demand monitoring, nor does he account for hourly variations in electrical cost. Harshbarger Jr.'s invention shuts down the heat pump when the ambient temperature drops below some predefined shutdown temperature.
Hines, et al. in U.S. Pat. No. 4,386,649 discloses a programmable thermostatic control device utilizing a microprocessor and an interface unit through which the heating, ventilation and cooling system of a building or residence are connected to device outputs. It is processor-based and is compatible with gas, fire, electric heating and cooling system as well as heat pump systems. Hines' system inhibits the auxiliary heat sources during recovery. The apparent intent is to use the system as efficiently as possible, and since the secondary source is electrical resistance heat, it is generally more efficient to operate the heat pump instead of the secondary source. This is not necessarily true for a dual fuel system. Hines does not minimize electrical demand cost nor does he predict heat and electrical loads or calculate a break-even demand set point.
The present invention provides advantages not offered in the aforementioned prior art for a dual fuel system including a prediction of monthly loads and a computation of break-even demands. The present invention has as a primary objective minimization of the total monthly bill to the energy consumer through the use of a unique implementation which integrates an intelligent demand logic algorithm into an operating thermostat.